(1) Field of the Invention
The present invention relates to a speed control apparatus for a bicycle. More particularly, the invention relates to a speed control apparatus for a bicycle comprising a fixed member, a take-up element for winding up a speed control cable, the take-up element being rotatably supported by the fixed member and urged in a cable unwinding direction, a wind-up control member operable to effect a change speed by rotating the take-up element against the urging force in the unwinding direction for winding up the cable, a position maintaining mechanism including a first engaging member and a first engaging portion engageable stepwise for maintaining the take-up element in a selected position, a rotation limiting mechanism including a second engaging member and a second engaging portion engageable stepwise for limiting rotation in the unwinding direction of the take-up element to a predetermined amount, and an unwind control member operable to effect a change speed by actuating the rotation limiting mechanism and releasing the take-up element from the position maintaining mechanism for unwinding the cable.
(2) Description of the Prior Art
A bicycle speed control apparatus as noted above is disclosed in U.K. patent publication GB-8432714 (Japanese Patent Publication Kokai No. 61-222884), for example. It is proposed in this prior publication to provide a rotation limiting mechanism for preventing the take-up element from rotating in the cable unwinding direction to the full extent due to the tension of the cable when the wind-up control member is operated to release the take-up element from the position maintaining mechanism. Specifically, the disk-shaped take-up element has top and bottom surfaces on which an engaging portion of the position maintaining mechanism and an engaging portion of the rotation limiting mechanism are formed as displaced a half pitch from each other. Pawls of the position maintaining mechanism and rotation limiting mechanism are formed integrally with upper and lower portions of an unwind control button urged axially upwardly of the take-up element. These pawls are opposed to the engaging portions, respectively.
However, since the pawls of the position maintaining mechanism and rotation limiting mechanism are formed integrally with the unwind control button according to the above known construction, these pawls are movable simultaneously with a pressing operation of the control button. Therefore, if the spacing between tips of the two pawls should become smaller than a predetermined value, the take-up element might be engaged by neither pawl during the operation of the control button. Then the take-up element would rotate continuously in the cable unwinding direction, thereby making change speed in the unwinding direction impossible.
The two pawls tend to wear to the extent to produce the above non-engagement state in a short period of time as a result of frequent change speed operations. It is thus difficult to promote durability of the speed control apparatus. In manufacture, it is necessary to set the spacing between the tips of the two pawls and the thickness of the take-up element with high precision. The requirement for strict dimensional precision tends to increase the manufacturing cost.